1. Field of the Invention
The present invention relates to a method for wireless data transmission between a base station and one or more transponders.
2. Description of the Background Art
Transmissions between one or more base stations or readers and one or more transponders can be found in, for example, contactless identification systems or so-called radio frequency identification (RFID) systems. Sensors, for example, for temperature measurement, may also be integrated in the transponders. Such transponders can also be referred to as remote sensors.
Transponders and their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply from an electromagnetic field, which is emitted by the base station.
In general, so-called backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, which the transmitting and receiving device in the transponder modulates and reflects appropriately for the data to be transmitted to the base station using a modulation method. The typical modulation methods for this purpose are amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is changed.
To transmit data from the base station to one or more transponders, symbols that are to be transmitted can be coded by the base station using a sequence of delimiters or so-called “notches.” The value of a symbol here is determined, for example, by the interval or the period between two sequential delimiters. For example, if the period is greater than a settable threshold value, the value of the symbol is “1,” otherwise it is “0.”
A variety of methods are known for producing the delimiters. In general, the carrier signal is amplitude-modulated and/or phase-modulated with a modulating signal by the base station. A delimiter is typically detected in a transponder using what is known as a receiver signal strength indicator (RSSI) circuit.
On-off keying (OOK) is a method based on amplitude modulation, in which the modulation signal is completely suppressed or blanked during a certain modulation period of the carrier signal. However, in this technique the modulated signal has a relatively broad-band spectrum. In passive systems, which take the energy required for their supply from the carrier signal, the energy supply is also completely suppressed during the modulation period, thus correspondingly reducing the achievable range. It is not possible to arbitrarily reduce the modulation period during which the carrier signal is blanked to any desired degree, however, since this increases the required bandwidth.
Methods for increasing the transmission range are known which do not completely blank the carrier signal during the modulation period, i.e., which have a modulation index that is less than one. However, this leads to a decrease in the reliability of transmission, since the delimiters can no longer be detected as reliably as with complete blanking.
To reduce the bandwidth required, the modulation signal can have a sinusoidal shape during the modulation period, i.e. the carrier signal is not blanked with a square-wave function, but instead is switched off and back on with a sinusoidal profile.
If a phase modulation method is used for modulation, the phase position of the carrier signal can be rotated by 180 degrees, for example, as in the method known as double sideband modulation (DSBM), to produce the delimiters. If this phase change during the modulation period takes place in a sinusoidal rather than an abrupt manner, the energy supply for the transponder can be improved in this way and the required bandwidth is reduced. The sidebands produced as a result of the sinusoidal change in the phase position limit the maximum transmission rate, however.